1. Field of the Invention
This invention relates generally to electrical discharge machining and, more particularly, to an improved method and apparatus for controllably delivering multiple sparks from a nonconsumable wire electrode to a workpiece.
2. Description of the Related Art
Electrical discharge wire cutting (EDWC) is a process that can be analogized to the operation of a bandsaw. In the case of EDWC, the "saw" is a wire electrode of small diameter that removes material by electrical erosion, thereby producing the appearance of the wire cutting through a workpiece. However, unlike the bandsaw, there is no actual contact between the wire and the workpiece, but rather, material removal is effected by spark erosion. Typically, a power supply has leads of opposite polarity connected to the workpiece and the wire respectively. Thus, when the wire and workpiece move sufficiently close to one another, the power supply voltage overcomes the resistance of air and current begins to flow through the air. This sudden manifestation of electrical current through air is directly observable as a spark. This same natural phenomena is observed daily on a much smaller scale as static electricity, or on a much larger scale as lightning. It should be appreciated that the greater the distance between the wire and workpiece, the greater the voltage necessary to overcome the resistance.
Further, the electrical erosion process generates large amounts of heat which should preferably be removed to prevent distortion of the workpiece. Air, unfortunately, does not have good heat transfer characteristics, but water does.
Deionized water is typically used to cool the workpiece because, like air, it only conducts electricity when the voltage drop across the spark gap is sufficient to ionize the water. Moreover, by concentrating the dielectric water into a high-velocity stream, the workpiece is not only cooled, but the removed material is effectively flushed away. Accordingly, the sparks actually occur in the deionized water media, rather than air.
These sparks carry sufficient current to locally heat the workpiece beyond its melting point. In fact, each spark vaporizes or melts a very small portion of the workpiece which is then flushed away by the constant stream of dielectric fluid. Thus, by continuously producing sparks, all of the workpiece material within a preselected spark gap distance from the wire is ultimately melted and flushed away. It can be seen that by advancing the wire through the workpiece, a kerf is produced in the workpiece much like that produced by a bandsaw. In most cases, movement of the wire and workpiece is controlled by a computer aided numerical control machine, which is capable of producing piece parts of complex and varied configurations.
EDWC is a process which is normally limited to unusual manufacturing operations that cannot be accomplished by more mainstream manufacturing tools (i.e. drills, punches, lathes, grinders, bandsaws, etc.). For example, EDWC is often used to cut hardened steel. Unlike the more conventional machine tools, EDWC is unaffected by the hardness of the material being cut. Thus, the material can be hardened prior to cutting and then electrically cut to a precise tolerance. Conversely, using conventional machine tools forces the manufacturer to harden the piece part after cutting. This post cutting hardening has serious repercussions, in that the piece part is distorted by the heat treating process.
While EDWC has achieved success in these specialized areas, the process has not realized widespread use. The cutting speed is relatively slow when compared to more conventional machine tools operating on nonhardened materials. Therefore, EDWC is not economically competitive with conventional machine tools. Accordingly, in order to expand EDWC into these more traditional areas, much attention has been focused on increasing the cutting rate of EDWC machines.
Unfortunately, prior systems have been generally unable to increase cutting speed without adversely affecting the quality (e.g. smoothness) of the cut. This is not totally unexpected since one obvious method of increasing cutting speed is to increase the volume of material removed by each spark. Clearly, if the energy delivered in each spark is increased, the degree of piece part heating will also increase, and the amount of material which is melted or vaporized must also correspondingly increase. However, material removal by spark erosion creates shallow, crater-shaped cavities. The depth of these cavities is proportional to the energy contained in the spark. Therefore, it can be seen that the greater the energy, the faster the cut, and the deeper the crater with a corresponding increase in surface roughness.
Accordingly, further increases in cutting speed were effected by increasing the rate at which sparks were delivered to the workpiece. The evolution of electrical discharge machining power supplies has generally been from the capacitive discharge type to pulse-type power supplies. In pulse-type power supplies, power is delivered directly to the wire electrode, resulting in a single high power discharge for each power supply pulse. However, as discussed previously, the amount of energy introduced by each spark must be limited to prevent excessive roughness and wire breakage. Further, it should also be recognized that each discharge not only melts or vaporizes a portion of the workpiece, but also produces the same effect on the wire electrode. Accordingly, the amount of energy need also be limited to prevent destroying the wire electrode.
Although it is generally true that the evolution has been toward pulse-type power supplies, it is also commonly recognized that capacitor discharge power supplies do offer some cutting advantages on certain materials. In fact, applicants have developed a capacitor discharge modification (FAST-TRACK I) to be used in conjunction with pulse type power supplies. This modification, in concert with other electronic, mechanical, and hydraulic modifications has a considerably faster cutting rate in most applications. The FAST-TRACK I unit has achieved a material removal rate in the range of 2-3 times greater than pulse-type EDWC. In the FAST-TRACK I capacitor type EDWC, the capacitor is periodically charged by high-power pulses from the power supply and produces numerous high-speed discharges between each periodic charging. Thus, the same energy that the pulse-type unit delivers in a single pulse is delivered by the FAST-TRACK I capacitor type unit in numerous, lower energy pulses. Therefore, more energy can be supplied in each charging pulse since it is dissipated over several discharges. With this higher energy input, the capacitor-type unit removes more material and cuts at a faster rate.
In FAST-TRACK the capacitor is selected to be of a size suitable for general purpose cutting and produces optimal discharge characteristics for a narrow range of materials. It is advantageous to select a capacitance value based upon the type of cut being performed, the material being cut, the thickness of the material, and other variables. Consequently, even while cutting a single piece part it is desirable to alter the value of the capacitance to the immediate conditions being experienced. Such "on the fly" adjustments to the capacitance are not possible on the FAST-TRACK unit.
Further, while FAST-TRACK effectively increases the number of discharges within a given time period, there is an upper limit to the frequency of these discharges. The discharge rate of the capacitor ultimately determines the frequency at which sparks are generated. It is desirable to further increase the spark generating rate to correspondingly increase the cutting speed without adversely affecting the quality of the cut.